net: dsa: mt7530: 1G can also support 1000BASE-X link mode

When using an external PHY connected using RGMII to mt7531 port 5, the
PHY can be used to used support 1000BASE-X connections. Moreover, if
1000BASE-T is supported, then we should allow 1000BASE-X as well, since
which are supported is a property of the PHY.

Therefore, it makes no sense to exclude this from the linkmodes when
1000BASE-T is supported.

Fixes: c288575f ("net: dsa: mt7530: Add the support of MT7531 switch")
Tested-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>